Smart ground-terminal antenna for geostationary satellites in slightly inclined orbits

ABSTRACT

A receive only smart antenna with a command pointing option for communicating with geostationary satellites that autonomously detects the directions from which desired signal are received, and steer the multiple beams accordingly. An array feed is used to illuminate a parabolic reflector. Each feed element of the receive only smart antenna is associated with a unique beam pointing direction. As a receiver is switched to different feed elements, the far-field beam is scanned, making it possible to track a geostationary satellite in a slightly inclined orbit. This eliminates the need for mechanical tracking and maintains high antenna gain in the direction of the geostationary satellite. The receive only smart antenna also features capabilities to form multiple simultaneous beams supporting operations of multiple geo-satellites in closely spaced slightly inclined orbits. The designs can support orthogonal beams for enhanced bandwidth capacity via multiple beams with excellent spatial isolation.

This application is a continuation of application Ser. No. 13/772,382,filed Feb. 21, 2013, now pending, which is continuation of applicationSer. No. 13/024,244, filed on Feb. 9, 2011, now U.S. Pat. No. 8,395,546.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ground-terminal antennas forcommunicating with satellites in geostationary orbit. More particularly,it relates to low-cost, electronically steerable antennas adapted tocompensate for motion of a satellite with respect to its fiducialgeostationary position, and to electronically steerable multi-beamantennas adapted to compensate for motions of multiple satellitessimultaneously.

2. Description of Related Art

Satellites in geostationary orbit are widely used for communications andbroadcast applications. When the orbit of a satellite lies along a path35,786 km directly over the equator, its orbital velocity exactlymatches the rate of rotation of the Earth, and the satellite remainsfixed in the sky relative to an observer on the ground. This greatlysimplifies the design of ground terminals because they can be designedto point in a single fixed direction and do not require bulky motorizedgimbals or tracking hardware. However, while a satellite ingeostationary orbit should theoretically remain at a fixed location inthe sky, perturbations to its orbit caused by interactions of the Sunand Moon as well as the non-spherical shape of the Earth itself causethe orbit of the satellite to drift away from its fiducial geostationarypoint. As shown in FIG. 1A, 1B, 1C, a satellite that drifts into aslightly inclined orbit with respect to the equator begins to trace outan elongated figure-eight pattern oriented in the north-south directionin the sky, as seen by the observer on the ground. This motion canresult in severe loss of signal by a ground terminal with a simple fixedantenna. A number of methods to address this problem have beendeveloped, but all have significant drawbacks.

One method of addressing this problem is to articulate the groundterminal by adding gimbals and a mechanical tracking system to allow theantenna pointing to be continually adjusted in order to track thesatellite. However, such a solution adds significant cost, bulk, andcomplexity and is not suitable for applications requiring a large numberof ground stations, such as direct-broadcast television.

Another method is to selectively broaden the antenna pattern in thenorth-south direction to account for the increased satellite motion inthis direction. For example, a typical one-meter-diameter parabolicantenna operating at Ku band will exhibit a beam width of approximatelytwo degrees. If the antenna reflector is compressed into an ellipse, thepattern in the north-south direction can be stretched totwelve-to-fourteen degrees, covering excursions of a satellite in anorbit inclined up to six or seven degrees with respect to the equator.However, stretching the radiation pattern significantly reduces antennagain, negatively impacting receive performance and requiring additionalpower for transmit.

Another method is to actively control the position of the satellite byfiring thrusters to perform “station-keeping” maneuvers in order to keepthe satellite as close as possible to the equator to minimizenorth-south excursions. The tighter the station-keeping requirementsimposed by the capabilities of the ground terminals, the more frequentare the required station-keeping maneuvers. When the satellite runs outof fuel, it can no longer be maintained in geostationary orbit, so thefrequency of such maneuvers directly affects the useful life of thesatellite.

Thus, it would be useful to provide a design for a low-cost, compact,ground terminal that does not require mechanical tracking and that wouldenable a relaxation of tight station-keeping requirements forgeostationary satellites in order to reduce fuel consumption and prolongtheir useful lifetimes.

In addition, it would be useful to provide broader bandwidth and higherdata rate to subscribers via a ground terminal design with multiplebeams that do not require mechanical tracking and that would enablemultiple satellites in slightly inclined orbits concurrently operatingin the same spectrum covering the same serve areas with no interferenceamong them.

SUMMARY OF THE INVENTION

A system is provided that autonomously detects a direction of arrival ofsignals from a geostationary satellite via diagnostic beams andgenerates both receiving (Rx) and transmitting (Tx) beams for mainsignals. The Rx system maintains high gain in the direction of thesatellite while it transmit signals are sent along the same directionback to the satellite. The ground terminal continuously tracks thesatellite motion without the need for a mechanical pointing system.

An embodiment of a retro-directive antenna terminal in accordance withthe present invention includes a parabolic reflector with an array feedpositioned near its focus. The array feed includes N feed elements,where N is an integer greater than or equal to two. An embodimentdescribed herein has N equal to four, providing a compromise between thecomplexity of the array feed and the pointing resolution of the antennabeam. However, an array feed comprising as few as two elements or morethan four elements would also fall within the scope and spirit of thepresent invention.

A parabolic reflector typically has a limited scan range, and far-fieldbeams arriving from directions that are a few degrees off of boresightwill focus at locations that are slightly offset from the boresightfocus of the antenna. Thus, energy arriving from off-boresight angleswill preferentially illuminate elements of the array feed that arepositioned slightly away from the reflector focus. Similarly, energyradiated from feed array elements that are located slightly off focuswill result in far-field beams that are pointed in directions a fewdegrees off of boresight. Thus, for a fixed boresight direction, anarray feed allows for electronic scanning of the antenna beam within thelimited scan range of the parabolic reflector. In the case ofgeostationary satellites that are inclined by a few degrees from theequator and thus move in a north-south direction relative to the groundterminal during the course of each day, a feed array oriented in thenorth-south, or elevation direction will allow the motion of thesatellite to be tracked without mechanically moving the boresightpointing direction.

An enhanced scan range in the azimuth direction can be achieved with anantenna reflector having a circular profile. Thus, a parabolic toroidalreflector having a parabolic profile in elevation and a circular profilein azimuth will exhibit a moderate scan range in elevation, as describedabove, combined with a wider scan range in azimuth. Such an antenna,equipped with an appropriate feed array, would be able to simultaneouslytrack multiple geostationary satellites separated in azimuth by over tenbeamwidths.

In order to maximize a reflector antenna gain for a beam position, itsassociated optimal feed dimensions for a beam in far field may beobtained by balancing the spill-over loss and the aperture efficiency.In the proposed applications in which continuous beam scanning isdesired, multiple contiguous elements will be utilized to function as asingle feed for optimized gain performance while supporting finepointing granularity with minimized cross-over loss. More than twoelements will be used for gain optimization of a beam position. Beamscanning are achieved through the following two configurations;

-   -   1. through switching matrix (and combiners) to achieve        “switching” a set of feed elements pointing to one beam        position, to another set of feed elements pointing to an        adjacent beam position;        -   a. majority of elements in the two sets are identical ones        -   b. there may only be one or two elements are replaced    -   2. through progressive phasing in a second (processing) domain        to achieve “switching” a set of feed elements pointing to one        beam position, to another set of feed elements pointing to an        adjacent beam position;        -   a. converting the antenna configuration from multiple beam            antenna to a “phased array” equivalent through Fourier            Transforming the Rx signals from the array of feeds        -   b. By properly amplitude and phase weighting on the            transformed signals, the contributing feed elements are            switched from one set to the second set.            In an embodiment of an antenna terminal in accordance with            the present invention, signals arriving at the N array feed            elements are individually amplified by low-noise amplifiers            (LNAs) and divided into two paths: a main receive (Rx) path            and a diagnostic path. The signals in the diagnostic path            are applied to the inputs of an N-by-N Butler Matrix (BM) or            other device configured to perform a spatial Fourier            transform (FT) of the array feed signals. We are using            configuration (1) for main beams and configuration (2) for            diagnostic beams.

Various inputs of the BM generate different phase progressions among theN outputs. The outputs of the Butler Matrix are then frequencydown-converted to form N baseband signals that are each digitized byanalog-to-digital converters. A direction-of-arrival processor thenmeasures the phase slope of the digitized signals to determine thedirection of the wavefront incident on the feed array elements and thus,the direction of arrival of the signal from the satellite. Thisinformation also enables the system to determine which of the feed arrayelements is being illuminated by the signal arriving from the satellite.

In the main receive path, the outputs of the LNAs are routed to a switchmatrix that is switched to select the illuminated feed array element asthe primary receive signal of the system. This signal may be frequencydown-converted and sent to the primary receiver of the system, whichmight be a digital television receiver or other communications device.For designs that may feature two or elements being highly illuminated,these elements will be combined properly in the switch matrix tofunction as a single feed.

A digital beam forming (DBF) processor uses the measured phase slopeinformation to calculate receive beam weight vectors (BWVs), which aresets of complex coefficients that can be used to adjust the amplitudeand phase of the signals from the elements of an array in order tocreate coherent beams pointing in selected directions. The receive BWVsoperate to index to proper transmit BWVs that are used to create atransmit beam that will be directed back along the direction of thereceive beam. Note that the correlation index of the receive (Rx)andtransmit BWVs is generated off line and beforehand as a look-up table toassure that the transmit and receive beams are always directed to andfrom the same feed element and thus pointed in the same direction.

Digital waveforms comprising the desired transmit signals to be sent tothe satellite are multiplied by the BWVs calculated by the DBF processorin order to create a set of N digital signals that exhibit a phase slopethat is conjugate to that of the received signals. These N digitalsignals are then routed through N digital-to-analog converters tosynthesize N analog baseband waveforms containing the transmit data andexhibiting the proper conjugate phase slope. The N analog basebandwaveforms are then frequency up-converted to N radio-frequency signals.These radio-frequency signals are amplified by solid-state poweramplifiers or other radio-frequency amplifiers known in the art and areapplied to a transmit-side Butler Matrix, or other device capable ofperforming a spatial FT. The outputs of the transmit-side Butler matrixare then applied to the feed array elements through diplexers, producinga transmit beam that is directed back along the line of sight to thesatellite.

In general, the receive beam can be thought of as being focused by theparabolic reflector onto one of the elements of the array feed. Thespatial FT then produces a set of signals encoding a phase slope that isindicative of the direction of the wavefront causing illumination ofthat array element. By encoding the conjugate of that phase slope intothe transmit signal and running it through a transmit-side FT, thetransmit energy appears preferentially at only one of the elements ofthe feed array. This then produces a beam that is retro-directed backalong the same line of sight as the received beam. Of course, it is alsopossible that the received beam will illuminate two of the elements ofthe feed array, indicating an arrival angle between those that wouldilluminate a single element. This would simply result in the transmitsignal's also being applied to the same two array feed elements toproduce a retro-reflected beam. It is conceivable to design a receivedbeam illuminating more than two elements of the feed array, indicatingan arrival angle among those that would illuminate a single undersizedelement of the feed array. A properly designed combining network isrequired to aggregate the received signals from these illuminatedelements functioning as a single receive element. This would simplyresult in the transmit signal's also being applied to the same arrayfeed elements with proper weighting in combining them to produce aretro-reflected beam.

Thus in the preferred embodiment, the beam formation is performed in a“wavefront domain.” The conversions to and from the beam domain takeplace in two spatial FT devices (the BMs). The phase progressions in thewavefront domain uniquely identify discrete signal directions associatedwith individual antenna feed elements. The reflector having multiplefeeds is characterized as a multi-beam antenna (MBA), and each of thefeed elements corresponds to a unique beam position in the far field.For a reflector with N feed elements, there are N distinct far-fieldbeam positions with associated beam widths. After processing by thespatial FT device, each of the N output ports receives signals from allof the N array feed elements simultaneously. The N output ports sharethe same field of view but have unique phase slopes associated with thedirections of arrival, similar to the characteristics of an arrayantenna.

It is also possible to implement retro-directive antennas in the “beamdomain” without the use of spatial FT devices. As compared to thewavefront-domain implementation described above that features gracefuldegradation in the transmit beam having N power amplifiers driving aspatial FT processor, the beam-domain implementation would feature aone-to-N switch matrix with a single large power amplifier at the inputside.

In an alternative embodiment of a retro-directed antenna terminal inaccordance with the present invention, orthogonal coding is used tosimplify the radio-frequency hardware and to eliminate the need formultiple down- and up-conversion stages and multiple A/D and D/Aconverters. In this alternative embodiment, the received signals fromfour array feed elements are routed through a Butler Matrix or otherradio-frequency FT processor. The four transformed outputs are thenrouted to four bi-phase modulators, each of which is driven by aseparate mutually orthogonal pseudonoise (PN) code produced by a codegenerator. The four modulated beams are then summed, and the compositebeam is frequency down-converted by a single down-converter anddigitized by a single A/D converter. The sampled composite beam is thenrouted through four matched filters, each of which correlates the samplestream with one of the PN sequences. Because of the mutual orthogonalityof the PN sequences, only that portion of the composite beam that wasoriginally modulated with the corresponding PN sequence will berecovered by the matched filter. Thus, this process allows for therecovery of four separate digital data streams while requiring only onedown-conversion chain and one A/D. This not only reduces RF parts countand complexity but also eliminates problems of poorly matched analogchannels that can degrade performance.

The digital samples are processed by a direction-of-arrival processor asbefore, and a DBF processor again calculates BWVs corresponding to themeasured phase slope and direction of arrival. In addition to using theBWVs to perform transmit-side beam forming, the BWVs are also used tomultiply the receive data streams coming out of the matched filters.This operation recovers the coherent sum of the feed array elements forthe direction of arrival of the wavefront from the satellite and thus isa fully digitized primary receiver signal that can be routed to theprimary receiver of the system, such as a digital television receiver orsimilar device. Thus, the need for a separate analog switch matrix andseparate down-conversion chain is also eliminated.

On the transmit side, the primary digital transmit waveform ismultiplied by the complex BWVs to create a phase profile that isconjugate to that of the receive side. The composite digital signal isthen routed to a single D/A converter that synthesizes an analogtransmit waveform encoded with the desired phase profile. This analogwaveform is then frequency up-converted to radio frequency. Theup-converted RF signal is then applied to four bi-phase modulatorsdriven by the same mutually orthogonal PN codes in order to create fourseparate modulated RF signals. These signals are then amplified bysolid-state power amplifiers or other RF amplification devices and areapplied to the four inputs of a transmit-side Butler Matrix or other RFFT processor. The constructive and destructive combinations that areformed inside the Butler Matrix then result in the output's beingdirected to the same feed element or elements of the array feed thatwere illuminated by the receive signal from the satellite.

The foregoing discussion described an embodiment of a retro-directiveantenna terminal having a four-element feed array. However, othernumbers of feed elements in the antenna feed array are possible withcorresponding adjustments to the number of inputs to the receive-sideand transmit-side Butler Matrices and other channel-specific hardware.Such systems would also fall within the scope and spirit of the presentinvention.

From the foregoing discussion, it is clear that certain advantages havebeen achieved for a retro-directive antenna terminal that autonomouslydetects a direction of arrival of a satellite signal and transmits backalong the same direction. Further advantages and applications of theinvention will become clear to those skilled in the art by examinationof the following detailed description of the preferred embodiment.Reference will be made to the attached sheets of drawing that will firstbe described briefly.

From the foregoing discussion, it is clear that certain advantages havebeen achieved for a retro-directive antenna terminal that autonomouslydetects a direction of arrival of a satellite signal and transmits backalong the same direction. It is also clear that

-   -   1. Rx only smart antennas featuring the same configurations        using diagnostic beams for detections of arrival directions of        desired signals to guide the pointing directions of the main        beam.    -   2. Rx only smart antennas featuring multiple orthogonal beams        with the same configurations using diagnostic beams for        detections of arrival directions of desired and undesired        signals to guide the peak pointing and null-steering directions        of the orthogonal beams concurrently.        -   a. A satellite ground terminal with orthogonal beams (OB)            exhibits the concurrent multiple beam; each beam pointing            its peak gain to a designated satellite while steering its            nulls to the peaks of all other OBs which are pointed            respectively to other satellites near by.            Further advantages and applications of the invention will            become clear to those skilled in the art by examination of            the following detailed description of the preferred            embodiment. Reference will be made to the attached sheets of            drawing that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B depict the orbital motion of a typical geostationaryorbit around the Earth

FIG. 1C depicts the orbital motion as viewed from directly below anobject in geostationary orbit

FIG. 2A illustrates a block diagram of an embodiment of a multiple-beamretro-directive ground terminal in accordance with the presentinvention;

FIG. 2B illustrates a block diagram of an embodiment of an Rx only smartground terminal in accordance with the present invention;

FIG. 2C illustrates a block diagram of an embodiment of a multiple-beamRx only smart ground terminal in accordance with the present invention;

FIG. 2D illustrates a block diagram of an embodiment of an Rx only smartground terminal with a command pointing option in accordance with thepresent invention;

FIGS. 3A and 3B are schematic diagrams illustrating grouping of antennafeed elements to achieve finer pointing resolution;

FIG. 4A illustrates a block diagram of an alternative embodiment of amultiple-beam retro-directive ground terminal in accordance with thepresent invention;

FIG. 4B illustrates a block diagram of an alternative embodiment of a Rxonly smart ground terminal in accordance with the present invention;

FIG. 4C illustrates a block diagram of an embodiment of a multiple-beamRx only smart ground terminal in accordance with the present invention;

FIG. 4D illustrates a block diagram of an alternative embodiment of amultiple-beam Rx only smart ground terminal with a command pointingoption in accordance with the present invention;

FIGS. 5A and 5B are schematic drawings of two embodiments of antennas inaccordance with the present invention showing single and multiplesatellite-tracking capability; and

FIGS. 6A and 6B depict the azimuthal scanning capability of a parabolicand a parabolic-toroidal antenna reflector in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides a simple, low-cost, limited-scan-angle,retro-directive antenna featuring an array feed capable of steering theantenna pattern to track orbital excursions of a geostationary satellitein an orbit inclined with respect to the equator by several degrees. Inthe detailed description that follows, like element numerals are used toindicate like elements appearing in one or more of the figures.

FIGS. 1A, 1B, and 1C depict the motion of a typical geostationarysatellite 106 in orbit around the Earth 102. The ideal geostationaryorbit 108 lies directly above the Earth's equator and results in thesatellite's appearing stationary in the sky with respect to an observeron the ground. Due to gravitational perturbations, the actual orbit 104of the satellite drifts, becoming inclined by up to several degrees withrespect to the equator. Periodic station-keeping maneuvers areundertaken to bring the actual orbit back toward an inclination of zerodegrees. The inclined orbit 104 crosses the equatorial plane at nodes110. FIG. 1A depicts the orbit from a direction along a line connectingthe two nodes 110. FIG. 1B depicts the orbit from a directionperpendicular to a line connecting the two nodes 110.

FIG. 1C depicts the apparent motion of the satellite 106 as viewed fromthe ground during the course of one day. The satellite 106 traces out afigure eight in a north-south direction, appearing at location 112 as itpasses the orbital nodes 110. The height of the figure eight depends onthe inclination of the orbit 104 with respect to the equator. Themajority of the satellite displacement is in the elevation direction;the magnitude of the displacement in azimuth is generally an order ofmagnitude smaller.

FIG. 2A depicts a block diagram of an embodiment of a ground terminal inaccordance with the present invention. The terminal includes a parabolicreflector 202 that is illuminated by four feeds 204, which may be horns,patches, or any other types of antenna feeds known in the art. The feedsare oriented to have the optimal polarization response individually, andthey are positioned linearly in the focal plane along a line parallel tothe local elevation direction. The scan range of a typical parabolicreflector is approximately +/−5 beamwidths. For a one-meter reflector atKu band, the beamwidth is approximately two degrees, and the scancapability is approximately +/−10 degrees. Signals arriving fromdirections within this scan range will be focused at slightly varyinglocations. Conversely, feeding the antenna from locations that varyslightly will result in antenna beams that point in slightly differentdirections within the scan range of the reflector. Thus, feeding theantenna from different elements or combinations of different elements ofthe feed array will result in beam steering in the far field. AlthoughFIG. 2A depicts a system having four feeds, other systems are possiblethat include N feeds, where N is an integer greater than or equal totwo, and such systems would also fall within the scope and spirit of thepresent invention.

Satellite signals impinging on the reflector 202 are focused onto thefeeds 204, are amplified by low-noise amplifiers 138, and are routed toa four-by-four Butler Matrix (BM) 212. The four-by-four BM includes four90-degree hybrids and two fixed phase shifters configured in a mannerwell known in the art to produce an output that is a spatial Fouriertransform (FT) of the input. The BM converts the beam-space signals fromthe feed elements to wavefront-domain signals. The four output wavefrontsignals from the BM are orthogonal to one another. Of course, forsystems including N feed elements, correspondingly sized N-by-N BMswould be used. The outputs of the BM 212 are then routed throughband-pass filters 214 and then to frequency down-converters 216 thatconvert the radio-frequency inputs to an intermediate or basebandfrequency. The frequency-down-converted signals are then digitized usinganalog-to-digital converters 218, and the digital samples are passed toa correlation processor 220. The correlation processor compares thedigitized samples from each of the BM outputs and calculates a phaseslope across the outputs. The direction-of-arrival (DOA) processor 222uses this phase slope to determine which of the antenna feeds within thefeed array is being illuminated by the signal from the parabolic orparabolic-toroidal dish 202. This information is then used in the mainreceive signal path to select the appropriate states of switches in theswitch matrix 206 in order to route the received signal from a selectedfeed to the primary frequency down-converter 208 in order to prepare theintermediate-frequency receive signal 210 that is routed to the mainreceiver (not shown). Methods of forming a spatial FT of the input otherthan using a Butler Matrix in the diagnostic path may also be used andwould fall within the scope and spirit of the present invention.

The direction-of-arrival information is also used by a digital beamforming (DBF) processor 224 to calculate an appropriate set of beamweight vectors (BWVs) that can be applied to the main transmit signal230 in order to select a phase slope that is conjugate to that of thereceived signal. When this phase slope is applied 228 to the transmittedbeam, it results in retro-directed transmit beam that propagates backalong the direction from which the received beam arrives. The maindigital transmit signal is multiplied 228 with the BWVs generated by theDBF processor 224, and the composite waveform is synthesized usingdigital-to-analog converters 226. The synthesized baseband waveform isthen frequency up-converted 232 and amplified 234 and applied to atransmit-side Butler Matrix 236. The outputs of the transmit-side BM arethen applied through diplexers to the antenna feeds 204 which illuminatethe reflector 202 and produce a retro-directed far-field beam. Note thatthe proper selection of the BWVs applied to the transmit signal 230 bythe DBF processor 224 results in constructive and destructive combiningthrough the transmit BM 236 to result in a non-zero output at only oneof the antenna feed elements 204—the same one upon which the receivesignal is incident. In other words, the selection of a set of BWVs atdigital baseband performs a switching function, directing RF energy tothe selected antenna feed element.

FIG. 2B depicts the same block diagram of the Rx portion of theembodiment of a ground terminal in FIG. 2A in accordance with thepresent invention. Satellite signals impinging on the reflector 202 arefocused onto the feeds 204, are amplified by low-noise amplifiers 138,and divided by two separated paths; one for main beam beam-forming andthe other for diagnostic. The signals for diagnostic are routed to afour-by-four Butler Matrix (BM) 212. The four-by-four BM produces anoutput that is a spatial Fourier transform (FT) of the input convertingthe beam-space signals from the feed elements to wavefront-domainsignals. The four output wavefront signals from the BM are orthogonal toone another. Of course, for systems including N feed elements,correspondingly sized N-by-N BMs would be used.

The outputs of the BM 212 are then routed through band-pass filters 214and then to frequency down-converters 216 that convert theradio-frequency inputs to an intermediate or baseband frequency. Thefrequency-down-converted signals are then digitized usinganalog-to-digital converters 218, and the digital samples are passed toa correlation processor 220.

The correlation processor compares the digitized samples from each ofthe BM outputs and calculates a phase slope across the outputs. Thedirection-of-arrival (DOA) processor 222 uses this phase slope todetermine which of the antenna feeds within the feed array is beingilluminated by the signal from the parabolic or parabolic-toroidal dish202. This information is then used in the main receive signal path toselect the appropriate states of switches in the switch matrix 206 inorder to route the received signal from a selected feed to the primaryfrequency down-converter 208 in order to prepare theintermediate-frequency receive signal 210 that is routed to the mainreceiver (not shown).

FIG. 2C depicts the same block diagram of a portion of the embodiment ofa ground terminal in FIG. 2B in accordance with the present invention.Both main signals and diagnostic processing are performed in wavefrontdomains.

Satellite signals impinging on the reflector 202 are focused onto thefeeds 204, are amplified by low-noise amplifiers 138, and are routed toa four-by-four Butler Matrix (BM) 212. The four-by-four BM produces anoutput that is a spatial Fourier transform (FT) of the input convertingthe beam-space signals from the feed elements to wavefront-domainsignals. The four output wavefront signals from the BM are orthogonal toone another. Systems with N feed elements feature N-by-N BMs.

The outputs of the BM 212 are then routed through band-pass filters 214and then to frequency down-converters 216 that convert theradio-frequency inputs to an intermediate or baseband frequency. Thefrequency-down-converted signals are then digitized usinganalog-to-digital converters 218, and the digital samples are passed toa correlation processor 220.

The correlation processor generates cross-correlations among thedigitized samples from the BM outputs and calculates a phase slopeacross the outputs for each of the desired signal sources. Thedirection-of-arrival (DOA) processor 222 uses these phase slopescomparing with pre-calibrated phase distributions of various beampositions, which are captured as different beam weight vectors (BWVs)for the parabolic or parabolic-toroidal dish 202 and specified feedconfigurations 204.

A selected BWV is then used in the Rx DBF 223 to generate a main receivesignal. Multiple Rx DBF processing will utilize various BWVsindependently generating different beams concurrently.

FIG. 2D depicts the same block diagram as the embodiment of a groundterminal in FIG. 2C in accordance with the present invention, except anexternal beam controller 240 is connected to the DOA process 222 c.

Satellite signals impinging on the reflector 202 are focused onto thefeeds 204, are amplified by low-noise amplifiers 138, and are routed toa four-by-four Butler Matrix (BM) 212. The four-by-four BM produces anoutput that is a spatial Fourier transform (FT) of the input convertingthe beam-space signals from the feed elements to wavefront-domainsignals. The four output wavefront signals from the BM are orthogonal toone another. Systems with N feed elements feature N-by-N BMs.

The outputs of the BM 212 are then routed through band-pass filters 214and then to frequency down-converters 216 that convert theradio-frequency inputs to an intermediate or baseband frequency. Thefrequency-down-converted signals are then digitized usinganalog-to-digital converters 218, and the digital samples are passed tomultiple Rx DBF processors 223.

The correlation processor 220 with recorded cross-correlations among thedigitized samples from the BM outputs provide data on unbalancedamplitudes and phase variations among the multiple elements; and effectsof multi-paths; which are captured in a “vector modifier” duringcalibration processing. The vector modifier will be used to modify BWVsfor various beam positions in the DOA processor.

On the other hand the external beam controller will continuouslycalculate the BWVs for various beam positions and associated nullpositions based on time of the day, and the coordinate of a subscriber.For slow moving satellites in slightly inclined orbits, the desiredreception patterns from a subscriber terminal may need to be updatedevery few minutes. The direction-of-arrival (DOA) processor 222 willincorporate the BWVs from the external beam controller 240 and thevector modifier from the correlator processor 220 as modified BWV inputsto the dynamic data streams for new beam positions for various Rx DBFprocessors 223.

The 4 signals in digital format from the 4 A/Ds 218 are the other set ofinputs to the Rx DBF processors. Outputs from a Rx DBF processor 223 arethe dynamic signal stream from a Rx tracking beam pointed to a desiredsatellite with nulls at undesired satellites nearby. Multiple Rx DBFprocessing will utilize various BWVs independently generating differentbeams concurrently.

In the embodiment discussed above, scanning of the far-field beam may beperformed in four discrete beam positions, each position correspondingto one of the four feed element locations. However, because a BM is alinear device, it is also possible to vary the signal intensity acrossmultiple feed elements to provide finer scanning resolution. Forexample, FIGS. 3A and 3B depict possible groupings of adjacent antennafeed array elements that may be used to point the far-field beam indirections between those achieved by using a single feed element. FIG.3A depicts an embodiment of a four-element array in accordance with thepresent invention. The antenna feed elements 302, 304, 306, and 308 maybe driven one at a time in order to point the far-field beam in fourslightly different directions. Alternatively, elements 302 and 304 canbe driven together as indicated at 310 by applying linear combinationsof BWVs to the digital baseband transmit signal that result in drivingelement 302 and element 304. The resulting far field beam will point ina direction between the beams formed when either element 302 or 304 isdriven alone. Similarly, other adjacent combinations may be formed, suchas those indicated at 312 and 314.

FIG. 3B depicts an alternative embodiment of a feed array in accordancewith the present invention in which nine antenna feed array elements,320-336, are used. Similarly, combinations of adjacent elements, e.g.,340, 348, may be used to provide finer resolution than drivingindividual elements alone would achieve. Systems using N array elements,where N is an integer greater than or equal to two, would also fallwithin the scope and spirit of the present invention.

Although the above discussion focused on the transmit-side applicationof the feed array, the concept of grouping adjacent elements to increasethe pointing resolution is equally effective for the receive operation.Again, because the BM 212 is a linear device, a signal incident on theparabolic reflector 202 that illuminates more than one feed element,e.g., the combination 310, can be viewed as a linear combination of asignal that illuminates element 302 and one that illuminates element304. From this linear combination, the DOA processor 222 is able todetermine a direction of arrival that lies between those of each elementtaken individually.

The far-field radiation produced by the feed arrays depicted in FIGS. 3Aand 3B are linearly polarized. However, the techniques described aboveare equally applicable to circularly polarized radiation. If apolarizing device, such as one implemented using meander-line techniqueswell known in the art, is placed in front of the feed array, transmittedlinearly polarized radiation can be circularly polarized. Similarly,received circularly polarized radiation can be converted to linearlypolarized radiation before being collected by the feed-array elements.

FIG. 4A illustrates an alternative embodiment of retro-directiveterminal in accordance with the present invention. This embodiment takesadvantage of high-speed digital electronics to simplify theradio-frequency processing. Signals impinging on a parabolic reflector402 are focused onto an array feed 404. The detected power from eachfeed element 404 is routed through a low-noise amplifier 406 and sent toa four-input BM 408. It should be appreciated that systems with more orfewer array feed elements and corresponding BM inputs and outputs wouldalso fall within the scope and spirit of the present invention. Eachoutput of the BM 408 is then bi-phase modulated 410 by a pseudonoise(PN) code sequence generated by a code generator 430. Each output of thecode generator 430 is used to modulate a corresponding one of theoutputs of the BM 408, and the PN code sequences are mutuallyorthogonal. The modulated outputs of the BM are then summed together412, and the composite RF signal is frequency down-converted 413 andthen digitized using an analog-to-digital converter 414. As compared tothe embodiment described with reference to FIG. 2A, above, fourindividual down-conversion devices (e.g, 216) are consolidated into asingle down-converter 413, which allows for better channel matching andsimplification of the radio-frequency portion of the circuit, assumingthe processing power of the digital circuit is adequate. Also eliminatedfrom the embodiment of FIG. 2 is a separate analog receive pathincluding a switch matrix 206 and an independent frequencydown-converter 208 for producing the main receive signal channel. As thespeed of digital processing hardware increases and the cost decreases,systems will tend to move further toward the digital architecturedepicted in FIG. 4A.

The digitized samples from the A/D 414 are then passed through a set ofmatched filters that correlate the samples with each of the orthogonalcodes applied to the outputs of the receive BM 408. Because of themutual orthogonality of the PN code sequences, digital samplescorresponding to the four outputs of the BM are recovered. Adirection-of-arrival (DOA) processor 422 analyzes the four digitizedoutputs of the BM 408 and calculates a phase slope that enablescalculation of the direction of arrival of the input radio-frequencybeam. A set of beam weight vectors (BWVs) are calculated by a digitalbeam forming (DBF) processor 420 to correspond to this direction ofarrival. These directional weights are then applied 418 to the outputsof the matched filter 416 to produce the digital receive signal 450 thatis sent off to the main system receiver.

The main digital transmit signal 426 of the system is also multiplied424 by a corresponding set of BWVs calculated by the DBF processor 420to produce a phase slope that is conjugate to the phase slope of thereceived beam. The transmit signals, mixed with appropriate BWVs arethen digitally summed 428, and a baseband waveform is synthesized usinga digital-to-analog (D/A) converter 432. The baseband waveform isfrequency up-converted 434 to radio frequency and is then modulated 436by the same set of four orthogonal PN codes 430 to produce fourcomponent signals that are then filtered by band-pass filters 438,amplified 440 and applied to the inputs of a transmit-side BM 442. Theoutputs of the transmit-side BM 442 then drive the array feed elements404 through diplexers 444. The proper choice of BWVs applied to thetransmit signal produces inputs to the BM that are then combined in sucha way that, in general, only one output of the BM is non zero.

Of course, as described with reference to the embodiment pictured inFIG. 2A, it is also possible to group antenna feed elements to improvethe scan resolution, and in that case, more than one of the outputs ofthe transmit-side BM 442 could be non zero. The matching of the phaseslopes achieved by the DOA processor 422 and the DBF processor 420 thusenables the system transmit signals to be retro-directed with respect tothe received signals.

It should be appreciated that the systems described with reference toFIGS. 2A and 4A do not require a continuous receive signal in order todetermine how to point the transmit beam. Both systems can save thedirection-of-arrival information calculated by the DOA processor, e.g.,422, and use it to apply appropriate BWVs at a later time to thetransmit data stream.

FIG. 4B depicts the same block diagram of an Rx portion of theembodiment of a ground terminal in FIG. 4 in accordance with the presentinvention. The DOA processor 422 will calculate the pointing directionstoward a desired satellite in an inclined orbit in terms of localazimuth and elevation, or equivalent. The information is captured as theupdated BWV buffer 420 in a Rx DBF processor 430.

FIG. 4C depicts the same block diagram of the embodiment of a groundterminal in FIG. 4B, except forming multiple beams concurrently pointedto different satellites in accordance with the present invention. TheDOA processor 422 will calculate the pointing directions toward desiredsatellites in inclined orbits in terms of local azimuth and elevation,or equivalent. The information for a given satellite is captured as theupdated BWV buffer 420 in a Rx DBF processor 430.

FIG. 4D depicts the same block diagram of the embodiment of a groundterminal in FIG. 4C, except command inputs from an external beamcontroller 440 to identify the beam positions and associated nulls formultiple beams concurrently pointed to different satellites inaccordance with the present invention. The command pointing is depictedin the 440 and 441 blocks based on (1) where the terminal is located andhow it is oriented, and (2) time of the date to derive satellitepositions in inclined orbits. The external beam controller 440calculates the pointing directions toward desired satellites and nullsagainst undesired satellites in inclined orbits in terms of localazimuth and elevation, or equivalent. The information for a beamposition and associated nulls is captured by the updated BWV stored in abuffer 420 in a Rx DBF processor 430.

FIG. 5A depicts a schematic view of an embodiment of a parabolic antennain accordance with the present invention. The reflector 502 has aparaboloid surface and is illuminated by a linear feed array 504comprising four feed element aligned in the local elevation(north-south) direction. The beam from the satellite is indicatedschematically at 506. By switching the transmit drive signal to variouselements of the feed 504 as described previously, the beam can be madeto scan in the elevation direction as indicated at 508.

In another embodiment in accordance with the present invention andillustrated in FIG. 5B, the reflector has a parabolic-toroidal surfacethat is parabolic in the elevation direction and circular in the azimuthdirection. The feed 530 of this embodiment comprises four independentfour-element linear arrays, e.g., 522 and 524. Each of the four-elementarrays is positioned in the focal plane along a line in the azimuthdirection. The beams created by each of the four four-element feedarrays are shown schematically, e.g., 526 and 528. The displacement ofeach feed array along the azimuth direction creates a beam that isdeflected in azimuth from the boresight of the antenna 520. Eachindividual beam can also be scanned in the elevation dimension, e.g.,532, by controlling which element of the linear array 524 is driven.Thus, such a system effectively combines four elevation trackingstations into a single aperture and could be used to track fourindependent geostationary satellites in slightly inclined orbits as longas they were not spaced too far apart in azimuth.

FIGS. 6A and 6B illustrate the improved azimuthal scanning performanceof a parabolic-toroidal antenna over a parabolic antenna. FIG. 6Adepicts azimuth cuts of the antenna pattern of a parabolic antenna.Degrees off of boresight are plotted along the horizontal axis 608, andthe relative pattern intensity in dBi is plotted along the vertical axis606. Individual azimuth cuts, e.g., 604, are plotted as a function ofboresight angle. The depiction illustrates that the pattern of aparabolic antenna falls off by 5 dB at a scan angle of 25 degrees.

Toroidal reflectors feature better scanning characteristics in Azimuthdirection. It is possible to design toroidal reflectors with Azimuthalscanning ranges of ±10 to ±15 beamwidths; significant improvement toconventional parabolic reflectors.

FIG. 6B illustrates the same azimuth cuts for a parabolic-toroidalantenna with a circular shape in the azimuth dimension. The patterncuts, e.g., 612, are plotted as a function of boresight angle 616. As isevident from the FIG. 6B, the amplitude falls off by only about 1 dB atscan angles of 25 degrees, illustrating the improved scanningperformance of the toroidal reflector.

Thus, a retro-directive antenna is achieved that takes advantage of thelimited field-of-view presented by a parabolic reflector fed by an arrayfeed. Each feed element of the retro-directive antenna is associatedwith a unique elevation pointing direction of the beam in the far field.As the transmit energy is switched to different feed elements, thefar-field beam is scanned in elevation, making it possible to track ageostationary satellite in a slightly inclined orbit. Theretro-directive antenna is able to autonomously detect the elevationdirection from which a signal is received, and a direction-of-arrivalprocessor and a digital beam-forming processor are used to prepare atransmit beam that points back along the same direction. This eliminatesthe need for mechanical tracking and maintains high antenna gain in thedirection of the geostationary satellite.

A similar technique is applied in parallel in the azimuth direction tocreate a multi-beam retro-directive antenna that can track multiplegeostationary satellites simultaneously and independently. Aparabolic-toroidal reflector is preferentially coupled to an array feedcomprising multiple linear arrays, each of which is capable ofsupporting tracking in the elevation direction. The displacement of themultiple linear arrays in the azimuth direction creates independentsimultaneous beams that point in different azimuth directions, eachcapable of independently tracking motion in the elevation direction.Those skilled in the art will likely recognize further advantages of thepresent invention, and it should be appreciated that variousmodifications, adaptations, and alternative embodiments thereof may bemade within the scope and spirit of the present invention. The inventionis further defined by the following claims.

What is claims is:
 1. A method for communication, comprising: providingan antenna system including a reflector having a focus and a feed arraypositioned at or near the focus and having N feed elements; receiving Nfeed signals via the N feed elements, the N feed signals resulting fromillumination of the N feed elements by a target signal incident on thereflector from a slow-moving signal source; inputting the N feed signalsto an N-to-N Fourier Transform device; performing a spatial FourierTransform on the N feed signals to generate N output signals, via theN-to-N Fourier Transform device; computing a phase slope across the Noutput signals, via a correlation processor coupled to the N-to-NFourier Transform device; and determining a direction of arrival of thetarget signal based on the phase slope, via a direction-of-arrivalprocessor coupled to the correlation processor.
 2. The method of claim 1further comprising: generating a beam weight vector for a beamassociated with the direction of arrival; and applying the beam weightvector to the N output signals to generate a receive beam, via a digitalbeam forming processor.
 3. The method of claim 1, wherein providing theantenna system comprises positioning the N feed elements linearly in afocal plane of the reflector or slightly away from the focus of thereflector.
 4. The method of claim 1, wherein providing the antennasystem comprises positioning the N feed elements as a plurality oflinear arrays in a focal plane of the reflector or slightly away fromthe focus of the reflector.
 5. The method of claim 1, wherein providingan antenna system comprises providing the reflector with a parabolicsurface or a parabolic-toroidal surface.
 6. The method of claim 1further comprising determining a first feed element of the N feedelements based on the direction of arrival, via the direction-of-arrivalprocessor.
 7. The method of claim 6 further comprising selecting a firstfeed signal of the N feed signals as a receive target signal, the firstfeed signal being associated with the first feed element, via a switchmatrix coupled to the direction-of-arrival processor and the N feedelements.
 8. A method for communication, comprising: providing anantenna system including a reflector having a focus and a feed arraypositioned at or near the focus and having N feed elements; receiving Nfeed signals via the N feed elements, the N feed signals resulting fromillumination of the N feed elements by a set of target signals incidenton the reflector from respective slow-moving signal sources; inputtingthe N feed signals to an N-to-N Fourier Transform device; performing aspatial Fourier Transform on the N feed signals to generate N outputsignals, via the N-to-N Fourier Transform device; computingcross-correlations from the N output signals to generate a vectormodifier, via a correlation processor coupled to the N-to-N FourierTransform device; computing a phase slope across the N output signalsfor each of the target signals, via the correlation processor;determining directions of arrival of the respective target signals basedon the respective phase slopes, via a direction-of-arrival processorcoupled to the correlation processor; and generating, based on thevector modifier and the directions of arrival, beam weight vectors forrespective beams associated with the respective directions of arrival,via the direction-of-arrival processor.
 9. The method of claim 8 furthercomprising: applying, via each of digital beam forming processors, arespective one of the beam weight vectors to the N output signals togenerate a respective receive beam; and outputting concurrently therespective beams from the respective digital beam forming processors asrespective received signals from the slow-moving signal sources.
 10. Themethod of claim 8, wherein providing the antenna system comprisespositioning the N feed elements linearly in a focal plane of thereflector or slightly away from the focus of the reflector.
 11. Themethod of claim 8, wherein providing the antenna system comprisespositioning the N feed elements as a plurality of linear arrays in afocal plane of the reflector or slightly away from the focus of thereflector.
 12. The method of claim 8, wherein providing an antennasystem comprises providing the reflector with a parabolic surface or aparabolic-toroidal surface.
 13. The method of claim 8 furthercomprising: inputting time of day and coordinates of a terminal thatincludes the antenna system to a beam controller; computing second beamweight vectors for a plurality of beam positions and associated nullpositions based on the time of day and the coordinates of the terminal,via the beam controller; inputting the second beam weight vectors to thedirection-of-arrival processor; and generating modified beam weightvectors based on the second beam weight vectors and the vector modifier,via the direction-of-arrival processor.
 14. The method of claim 8,wherein the N-to-N Fourier Transform device includes a Butler Matrix.15. A method for communication, comprising: providing an antenna systemincluding a reflector having a focus and a feed array positioned at ornear the focus and having N feed elements; receiving N feed signals viathe N feed elements, the N feed signals resulting from illumination ofthe N feed elements by a target signal incident on the reflector from aslow-moving signal source; inputting the N feed signals to a firstN-to-N Fourier Transform device; performing a spatial Fourier Transformon the N feed signals to generate N output signals, via the first N-to-NFourier Transform device; computing a phase slope across the N outputsignals, via a correlation processor coupled to the first N-to-N FourierTransform device; and determining a direction of arrival of the targetsignal based on the phase slope, via a direction-of-arrival processorcoupled to the correlation processor; generating a beam weight vectorfor a transmit beam associated with the direction of arrival; andapplying the beam weight vector to a transmit signal to generate Nelement signals, via a digital beam forming processor.
 16. The method ofclaim 15 further comprising: converting the N element signals fromdigital signals to analog signals via N digital-to-analog converters;frequency up-converting the N analog element signals; and amplifying theN up-converted analog element signals.
 17. The method of claim 16further comprising: inputting the N amplified up-converted analogelement signals to a second N-to-N Fourier Transform device; performinga second spatial Fourier Transform on the N amplified up-convertedanalog element signals to generate N output transmit signals, via thesecond N-to-N Fourier Transform device; and applying the N outputtransmit signals to the N feed elements via diplexers.
 18. The method ofclaim 15, wherein the first and second Fourier Transform devices arerespectively first and second Butler Matrices.
 19. The method of claim15, wherein providing an antenna system comprises providing thereflector with a parabolic surface or a parabolic-toroidal surface. 20.The method of claim 15, wherein providing the antenna system comprisespositioning the N feed elements linearly in a focal plane of thereflector or slightly away from the focus of the reflector.